Playability, which is primarily characterized by the force required to press down the strings, is a major criterion for the quality of musical instruments of the above-mentioned type. First of all, this string pressdown force depends on the distance between the lower edge of the string and the upper edge of the fingerboard, or, with fretted instruments, the frets.
The term "fingerboard surface" shall denote herein the surface profile of the fingerboard, both for fretted instruments (such as guitars) and fretless instruments (such as stringed instruments, but also fretless electric bass guitars), no matter whether fretwires are inwrought or not. In top view, most fingerboards have a trapezoid plane as they get wider towards the body; this plane is found, for example, in classical guitars. But they may also have a cambered surface as with stringed instruments, electric and Western guitars; in this case, their three-dimensional shape corresponds to a patch of the lateral surface of a truncated cone.
Although this description focuses on measuring and processing fretted instruments (e.g. guitars) for convenience and clarity, the facts described apply likewise to fretless instruments, only that with the latter the string does not strike on the frets but on the wood of the fingerboard if, for example, the distance is too small.
However, the distance between the strings and the frets cannot be reduced to any small measure to keep the string pressdown force to a minimum as the vibration of the string may be impeded by its striking on the nearest following frets. The neck of the instrument or a line across its fret surfaces has to be slightly curved to give a vibrating string the space required at each point of the fingerboard. There is only one optimum fingerboard profile for each string of an instrument at a constant temperament. Any deviation from this line means that either the string is positioned too high above the fingerboard, which impedes playability, or it is positioned too low and strikes on the nearest following frets (or on the fingerboard surface) when vibrating.
Unless prevented by the design or material properties of, the neck of some instruments can be deflected due to the tensile stress exerted by the strings. Such deflection, on the one hand, is not uniform because the neck of the instrument thickens towards the body, and because the end of the fingerboard is directly glued onto the sound board (or onto the solid body, for example, of an electric guitar); on the other hand, it can hardly be predetermined due to the specific properties of the material wood. The result are composite curve sections with different curvatures. Optimum curvature adjustment using the neck adjusting screw that some instruments have (it counteracts the tensile stress exerted by the strings inside the neck) is also impossible due to the varying thickness of the neck.
Another problem is the fact that the distance of the fret surfaces from the wood of the fingerboard surface is not uniform, which is caused by the manufacturing process (manual hammering or pressing in of the fretwires) and by fret wear and tear when the instrument is played. The mechanized and the manual manufacturing process both cause deviation in the relevant range (&gt;0.02 mm). This is traditionally taken into account by manual grinding (so-called tuning) of the fret surfaces. The deflection caused by the tensile stress exerted on the neck by the strings cannot be taken into account in this process because the strings have to be removed and no precise values are known as regards the quantity of material to be ground off from the fret surfaces.
The mode of vibration of the string also influences the required curvature of the fingerboard. It mainly depends on the properties of the string material (steel/nylon, diameter, bare/covered, tensile stress, diapason length, etc.) and on the force with which the vibration is excited (temperament). As the various strings of an instrument differ in gauge and have different tensile stresses, the fingerboard curvature has to vary along each individual string. The required fingerboard profile of heavier-gauge and less tightened strings has a sharper curvature in line with their vibration envelope. Therefore the curve is the flatter the thinner the string and the greater its tensile stress. Moreover, the required fingerboard curvature increases (or decreases) as a function of temperament in proportion to the larger (or smaller) oscillation amplitude of the string.
These differences in the vibration behaviour of the individual strings is not taken into account in conventional fingerboard processing as no binding values are known and the accuracy required for curving the fingerboard individually for each string (+/-0.02 mm) in order to give the fingerboard along each string an individual curvature cannot be achieved by manual tuning. Conventional tuning of the fingerboard by which the upper fretwire edges are manually ground to height in a curved plane is limited in effectiveness.
It is therefore the problem of this invention to provide a method for optimizing the individual position of the strings on stringed and plucked instruments to achieve good playability that does not depend on manual skills and does not take much time, and to provide a corresponding device for carrying out this method.